Laterite ores are oxidised ores and their exploitation requires essentially whole ore processing as generally there is no effective method to beneficiate the ore to concentrate the valuable metals nickel and cobalt.
As shown in Table 1, the iron/nickel ratio is variable being high in the limonite fraction and lower in the saprolite fraction, therefore the separation of solubilized nickel and cobalt from dissolved iron is a key issue in any recovery process.
TABLE 1Iron, Nickel and Cobalt Content in Various Laterite Ore SampleFeMgNiCoFe/NiOre Typewt. %wt. %wt. %wt. %ratioIndonesia limonite40.81.301.530.1027Indonesia saprolite8.514.603.370.033Indonesia saprolite with high Fe18.511.102.180.149contentNew Caledonia limonite47.10.401.330.1635New Caledonia saprolite7.723.31.000.028Western Australian low-Mg ore25.44.902.500.0710Western Australian high-Mg ore10.016.61.380.027Cuban low-Mg nontronite ore21.62.601.800.0512Cuban high-Mg nontronite ore18.88.301.170.0416
In the acid leaching of lateritic ore, the high pressure acid leaching (HPAL) process was developed to dissolve nickel and cobalt and convert the major portion of solubilized iron to insoluble hematite. This was achieved in autoclaves operated at high temperatures (250-300° C.) and associated pressures. HPAL methods recover high percentages of nickel and cobalt but require expensive, sophisticated equipment to withstand the high pressure and temperature operating conditions.
In order to avoid the use of expensive equipment, alternatives to the HPAL process have been disclosed. These generally operate at temperatures up to 110° C. at atmospheric pressure. One such disclosure is U.S. Pat. No. 6,261,527, which describes the sequential leaching of limonite and saprolite fractions of laterite ore with sulphuric acid at atmospheric pressure and temperatures below the boiling point, discarding most of the dissolved iron as insoluble jarosite solids.
There are environmental concerns with this iron removal process as the jarosite compounds are thermodynamically unstable. Jarosite may decompose slowly to iron hydroxides releasing sulphuric acid. The released acid may redissolve traces of precipitated heavy metals, such as Mn, Ni, Co, Cu and Zn, present in the leach residue tailing, thereby mobilizing these metals into the ground or surface water around the tailings deposit.
Another disadvantage of this process is that jarosite contains sulphate, and this increases the acid requirement for leaching significantly. Sulphuric acid is usually the single most expensive input in acid leaching processing, so there is also an economic disadvantage in the jarosite process.
U.S. Pat. No. 6,379,637 in the name of Walter Curlook describes an atmospheric acid leach process for leaching nickel and cobalt from highly serpentinized saprolitic fractions of nickel laterite ores. This process involves the leaching of the highly serpentinized saprolitic ore by the direct addition of sulphuric acid solutions to the ore at atmospheric pressure. The acid consumption in this process is suggested to be 800 to 1000 kg per tonne of dry ore.
UK Patent GB 2086872 in the name of Falconbridge Nickel Mines Ltd, relates to an atmospheric leaching process of lateritic nickel ores whereby nickel and cobalt are solubilized from high-magnesia nickelferous serpentine ores by leaching the ore with an aqueous solution of sulphuric acid. A reducing agent is also added to the solution in large quantities to maintain the redox potential of the solution at a value of between 200 and 400 mV measured against the saturated calomel electrode.
Such processes utilize direct addition of acid in the leaching process where acid is used to leach the whole content of the ore being processed. With sulphuric acid being an expensive input in the acid leaching process there are economic as well as environment disadvantages to such processes.
The present invention aims to overcome or alleviate one or more of the problems associated with prior art processes.
The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed in Australia before the priority date of each claim of this application.